In recent years, thin client server has been attracting public attention from the viewpoint of enhancing security and desk-top portability. On the thin client server based on virtual machines (VM) (VirtualPCCenter and so on) in corporate data centers and so forth, paid software such as Microsoft Office (registered trademark) is supposed to run.
In this sort of environment, it is anticipated that a VM image is duplicated by a malicious user or system administrator, and leaked to a large indefinite number of people. This is because digital contents are readily copied or falsified, and may readily be spread broadly through networks, while being assisted by dissemination of file conversion techniques (such as P2P (Peer to Peer)).
It is therefore necessary to trace the leakage, if the digital contents such as VM image including the paid software were illegally leaked. In recent years, techniques for tracing the digital contents have been developed (see Non-Patent Documents 1 and 2, for example).
Embedding of digital watermark into the digital content is one possible technique of tracing the digital content. The digital watermark is a technique of embedding identification information or the like into the digital content, without largely degrading the quality.
Even if the digital content illegally leaks as a result of breakage of tamper-proofing technique or cryptographic technique as a block against license violation, the leaked digital content is traceable by using the digital watermark.
In recent years, techniques of tracing the digital content have been developed (see Non-Patent Document 1, for example). For digital contents relevant to pictures and sounds, a large number of digital watermark techniques have been developed. According to Non-Patent Document 1, those making use of frequency range or picture replacement are exemplified for picture contents, and those making use of echo or phase modification are exemplified for sound contents.
A known technique of embedding a digital watermark into an picture content is described in Patent Document 1. Moreover, a method of embedding a file name of a content into the content as a part of ID information is described in Patent Document 2.